High-frequency power generators are used for the power supply of plasma-generating loads, for example, plasma chambers for etching or sputtering, or CO2 power lasers, as well as for the voltage supply of coils of industrial heating systems. High-frequency power generators can generate an output voltage with a frequency, for example, between 50 kHz and 50 MHz and an output power, for example, of up to several kW. The high-frequency power is often generated by a quartz oscillator or by an oscillator stabilized by an oscillating circuit in the small signal range and is amplified in downstream amplifier stages in order to be then amplified to the demanded power level. A class E amplifier is often used for amplification in the amplifier stages.
The operation of a typical E amplifier is described, for example, in U.S. Pat. No. 3,919,656 and is explained in more detail with reference to FIG. 15. The configuration of such a class E amplifier includes, in principle, a switching element 3, a capacitor 4 in parallel with the switching element 3, a voltage source 1 that is connected to the switching element 3 and the capacitor 4 by a high-frequency choke 2, as well as a series oscillating circuit 10. The high-frequency choke 2, by which the amplifier is connected to a supply voltage, is large enough to ensure a constant current during the period of the fundamental wave. The series oscillating circuit 10 is tuned to the fundamental frequency and only allows a sinusoidal characteristic of the current to reach the load resistance 9. An additional reactive element 7 generates a phase shift, by means of which the switching conditions for the switch are adjusted. The turn-on point of the switch 3 should be chosen such that the voltage at the capacitor 4 is preferably zero, to avoid losses due to discharging of the capacitance via the switch 3. Additionally, it is advantageous if the voltage variation is as small as possible for a long period of time for the losses due to a finite turn-on time of the real switch to remain as small as possible.
As described in U.S. Pat. No. 3,919,656, the series resistance, which represents the load 9, and the series capacitor 5 are converted by means of known impedance transformation into a parallel circuit with a first capacitor to ground and a series circuit of a second capacitor and a resulting load resistor, which also lies at ground. At the resulting center tap of the two capacitors the inductor 6 is connected. Such a circuit results in a comparable behavior to that of the circuit according to FIG. 15 at the one frequency, for which this impedance transformation is performed, however, for other frequencies the circuit behaves differently. Thus, U.S. Pat. No. 3,919,656 shows equivalent circuits for the series oscillating circuit 10, which have a similar function at the fundamental frequency to the series oscillating circuit.
In the classical class E amplifier according to FIG. 15 a resistance transformation occurs. To avoid reflections and losses, the resistance of the load 9 should equal the internal resistance of the amplifier 8. In the technical field typical loads are approximately 50 Ω. The internal resistance of the amplifier 8, which is determined by the required power and the maximum voltage that is present at the switch 3, is normally smaller than 50 Ω. Among others, the devices used for the switch 3 limit the maximum value of the voltage, since they have a finite breakdown voltage.
FIG. 16 shows this class E amplifier with a resistance-matching network 11, which transforms the internal resistance of the amplifier 8 to the resistance 13, which is ideally equal to the resistance of the load 14. For matching to the required load resistance, as shown in FIG. 16, matching networks 11 may be provided, which are lossy and lead to reduction of the overall efficiency.